1. Field of the Invention
This invention relates to imaging devices for robot vision. More particularly, this invention relates to the sensing of polarized radiation emitted by objects and the determination of the shape and pose of objects.
2. Description of Prior Art
In order to recognize three-dimensional objects, conventional methods in robot vision perform shape extraction by sensing the intensity of light reflected by objects. A fundamental problem associated with sensing the intensity of reflected light is that intensity gives one parameter while the surface orientation of objects have two degrees of freedom. Several techniques have been developed to obtain from intensity measurements at least two parameters to establish surface orientation: Photometric stereo combines multiple images, each with a different arrangement of light sources. In shape-from-shading techniques, a single brightness image is used together with assumptions about the smoothness of the objects, and characteristic curves are traced on a reflectance map. The technique of photogrammetry uses stereo pairs of images to determine object shapes. A review of these techniques using intensity is given in "Robot Vision" by B. K. P. Horn, MIT Press, Cambridge, 1986. Other problems encountered with reflected intensity techniques are that they require controlled light sources and large amounts of computation.
A technique for measuring at least two parameters for determining surface orientation is to measure intensity, percent of polarization, and angle of polarization for infrared thermal emission from objects. A method and apparatus for making measurements of only intensity and percent of polarization is described in U.S. Pat. No. 5,138,162 to Hacskaylo (1992). In the referenced patent, a single, linear polarizer covers the entire imaging sensor and two images are taken. It is well known in the art that, using two orientations of a linear polarizer, one can measure intensity and percent of polarization, but not angle of polarization. In the Hacskaylo patent each image is stored in electronic memory with the polarizer rotated between images. Among the disadvantages of the technique in the referenced patent is that the polarizer must be mechanically rotated between images which would increase the risk of mechanical failure in an operational imaging system. Another disadvantage is that the position and orientation of objects must remain constant throughout the entire sequence of images. Hence the polarization information cannot be determined if the imaging system is vibrating or if the objects are rapidly moving. A further disadvantage is that images must be stored until the entire sequence of images is captured. High resolution images will occupy large amounts of electronic storage, and the rate that images can be processed will be limited by the speed for writing to and reading from the electronic storage.
An objective of the present invention is to provide a new and improved method and apparatus for determining intensity, percent of polarization, and angle of polarization in a field of view. Another objective of the present invention is to provide a new and improved method and apparatus for determining the surface orientation of objects in a field of view.
Another objective of the present invention is to provide a method and apparatus for determining the surface orientation of objects without requiring conditions of controlled lighting. Indeed, this invention can function in darkness because the apparatus detects the thermal radiation emitted by all objects.
A further objective of the present invention is to provide a method and apparatus for determining surface orientation of objects which may be moving rapidly relative to the imaging device.
Yet another objective of the present invention is to provide a method and apparatus for determining surface orientation of objects which eliminates the mechanical rotation of polarizers.
Yet a further objective of the present invention is to provide a method and apparatus for determining surface orientation of objects which increases the rate images can be processed by reducing the requirement for electronic storage of images.
Further objectives and advantages of the present invention will become apparent from a consideration of the drawings and the description of the preferred embodiments.